V4L/DVB (6707): pvrusb2: Remove use of volatile in pipeline control state machine

pvrusb2: Eliminate use of volatile in pipeline control state
variables.  These were all cases of paranoia; upon further review the
overall mechanism employed here should not require use of volatile.
This had originally been done out of paranoia, and I have since been
convinced that the paranoia is not required.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>